Free Thyroxine (FT4) Unit Converter
Free Thyroxine (FT4) Unit Converter
Convert free T4 between ng/dL, pmol/L and ng/L, and see why the interval belongs to the assay rather than to the patient.
Free Thyroxine (FT4) converter
Mass ⇄ molarFree T4 1.2 ng/dL
Formula and conversion factors
ng/dL = pmol/L ÷ 12.8722
ng/L = ng/dL × 10
- 12.8722
- derived from the molecular mass of thyroxine, 776.87 Da
- ng/L
- ten times the ng/dL figure, because a decilitre is 100 mL
- free fraction
- only about 0.03% of total T4 circulates unbound; the rest is carried on thyroxine-binding globulin, transthyretin and albumin
- assay-specific
- free T4 immunoassays estimate the free fraction rather than measuring it directly, so the reference interval belongs to the method, not to the analyte
Worked example
Free T4 1.2 ng/dL
1.2 × 12.8722 = 15.4 pmol/L
1.2 × 10 = 12.0 ng/L
Within the adult reference interval of 0.8–1.8 ng/dL
Free T4 across the three units
| ng/dL | pmol/L | ng/L |
|---|---|---|
| 0.50 | 6.4 | 5.0 |
| 0.80 — typical lower limit | 10.3 | 8.0 |
| 1.20 | 15.4 | 12.0 |
| 1.80 — typical upper limit | 23.2 | 18.0 |
| 3.00 | 38.6 | 30.0 |
Why free T4 and not total T4
| Situation | Binding proteins | Total T4 | Free T4 |
|---|---|---|---|
| Pregnancy, combined oral contraceptive, oestrogen therapy | Raised | Raised | Unchanged |
| Nephrotic syndrome, protein-losing enteropathy | Lowered | Lowered | Unchanged |
| Androgens, high-dose corticosteroids | Lowered | Lowered | Unchanged |
| Inherited TBG excess or deficiency | Raised or lowered | Follows the binding protein | Unchanged |
The free fraction, and why the interval belongs to the assay
Free T4 is reported in ng/dL in the United States and in pmol/L almost everywhere else. The factor between them, 12.8722, comes from the molecular mass of thyroxine, 776.87 daltons, so a free T4 of 1.2 ng/dL is 15.4 pmol/L. A third convention, ng/L, is simply the ng/dL figure multiplied by ten. The arithmetic is straightforward; what needs care is what the number represents and how far it can be compared between laboratories.
Only about 0.03 per cent of circulating thyroxine is unbound, the rest being carried on thyroxine-binding globulin, transthyretin and albumin, and it is that tiny unbound fraction that enters cells and drives the pituitary feedback loop. Anything that changes the binding proteins therefore moves the total T4 without moving the free T4. Pregnancy, the combined oral contraceptive and oestrogen therapy raise binding globulin and so raise the total; nephrotic syndrome, protein-losing enteropathy, androgens and high-dose corticosteroids lower it and lower the total. In each case the patient is euthyroid and the total T4 is misleading, which is why the free hormone is the measurement used.
Free T4 assays are estimates rather than true measurements of free hormone. They are one-step or two-step immunoassays whose results depend on the binding equilibrium in the reaction, and they disagree appreciably between manufacturers. Reference intervals are consequently assay-specific, and a patient who moves between laboratories can appear to change more than the patient actually has. Serial values are meaningful only within one method, and a result should always be read against the issuing laboratory’s own interval.
Pregnancy is the situation where this bites hardest. Rising binding globulin and the weak thyrotropic action of hCG shift the free T4 interval downward, most obviously in the first trimester, so applying a non-pregnant range will label normal women as hypothyroid or miss a genuine fall. Trimester-specific and, where possible, assay-specific ranges should be used, and free T4 should be interpreted alongside TSH rather than on its own.
Frequently asked questions
How do I convert free T4 from ng/dL to pmol/L?
Multiply by 12.8722, a factor derived from the molecular mass of thyroxine, 776.87 Da. A free T4 of 1.2 ng/dL is 15.4 pmol/L. To go the other way, divide the pmol/L figure by 12.8722.
Is ng/L the same as ng/dL?
No. A decilitre is 100 mL, so ng/L is ten times the ng/dL figure. A free T4 of 1.2 ng/dL is 12 ng/L, and confusing the two produces a tenfold error.
Why measure free T4 rather than total T4?
Only about 0.03% of T4 circulates unbound, and that fraction is what acts on tissues. Pregnancy, oestrogen and nephrotic syndrome change the binding proteins and so change the total T4 in a euthyroid patient, while the free T4 stays put.
Can I compare free T4 results from two different laboratories?
Not reliably. Free T4 immunoassays estimate the free fraction rather than measuring it, and different manufacturers’ methods disagree. Reference intervals are assay-specific, so a change of laboratory can shift a result more than a change in the patient.
Does the free T4 range change in pregnancy?
Yes. The interval shifts downward, most clearly in the first trimester, because binding globulin rises and hCG has weak thyrotropic activity. Trimester-specific and ideally assay-specific ranges should be used.
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References
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315–389.
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism. Thyroid. 2014;24(12):1670–1751.
- Thienpont LM, Van Uytfanghe K, Beastall G, et al. Report of the IFCC Working Group for Standardization of Thyroid Function Tests; part 2: free thyroxine and free triiodothyronine. Clin Chem. 2010;56(6):912–920.